1. Field of the Invention
The present invention relates to a directional receiver contained in a handheld portable housing that is useful at VHF frequencies and on FM modulated signals. This invention is intended primarily for use above 100 MHz and is relatively insensitive so that it can be used close to the transmitting antenna without overloading. It would normally be used within sight of the transmitting antenna to determine precisely where or which is the antenna in use.
2. Description of the Prior Art
In the field of portable directional receivers, it has been the general practice in the art to employ a ferrite rod loopstick antenna to find the RF signal source by rotating the antenna until a null is obtained in the signal level. Such receivers are not effective above approximately 50 MHz and have been capable only of detection of Amplitude Modulated signals.
Specifically, Warren, U.S. Pat. No. 3,683,384, utilizes the typical loopstick and limits his demodulation capability by incorporating a frequency filter that would disrupt any intelligence on the signal, passing only the 1020 Hertz tone present on the radio beacons in the 300 kHz region of the spectrum. Leeming, U.S. Pat. No. 3,115,635 and Held, U.S. Pat. No. 3,323,129 likewise utilize the typical loopstick with its frequency limitations and a detector for AM demodulation only.
The closest device in the field of direction finding to this invention is the Leigh DRF-2. Although this device will DF in the VHF frequency range there are several major differences. The DRF-2 was intended as a receiver to be used with a transmitter emitting a continuous low power signal on a specific frequency. Specifically, it was intended to locate downed aircraft by finding the crash activated ELT (Emergency Locating Transmitter) required on all FAA approved aircraft. The Leigh utilizes two vertical antennas approximately 12 inches in height that extend from the top of the receiver to make an overall height approaching 20 inches. This invention on the other hand, is only 41/2 inches high including the complete receiver, battery pack, and antenna and will fit comfortably in a coat pocket. The Leigh device is designed to be able to receive a one-half watt signal at a range of several miles and not overload at close range only on a signal of this low power. This invention is designed to receive a one watt signal no further away than a few hundred feet and will not overload on a twenty-five watt signal even when ten feet away. The Leigh device is also limited to AM demodulation and could not be utilized discreetly in a crowded environment.